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In biology, an '''organ''' or '''viscus''' is a collection of Tissue (biology)|tissues joined in a structural unit to serve a common Function (biology)|function. In anatomy, a '''viscus''' is an internal organ, and '''viscera''' is the plural form. Organs are composed of main tissue, parenchyma, and "sporadic" tissues, stroma (animal tissue)|stroma. The main tissue is that which is unique for the specific organ, such as the myocardium, the main tissue of the heart, while sporadic tissues include the nerve tissue|nerves, blood vessels, and connective tissue|connective tissues. Functionally related organs often cooperate to form whole organ systems. Organs exist in all higher biological organisms, in particular they are not restricted to animals, but can also be identified in plants. In unicellular|single-cell organisms like bacteria, the...
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